Sintered part and the method for production thereof

ABSTRACT

The invention relates to a sintered part consisting of a hard metal, in particular WC with Co, Ni and/or Fe binders-based hard metal, or cermet, in particular on the base of composition consisting of (Ti, W) (C, N) or (Ti, Mo) (C, N) with Co, Ni and/or Fe binders and to a method for producing such a sintered part. The inventive method consists in exposing a sintered part totally or partially to an active plasma gaseous phase at a maximum pressure of 3×10 4  Pa during heating, sintering or after sintering, at least during a certain time preferably during a time period ranging from 10 to 100 min.

The invention relates to a sintered body comprised of a hard metal, especially a hard metal on a basis of WC containing Co, Ni and/or Fe binder proportions, or upon a cermet, especially on the basis of a (Ti,W) (C,N) or (Ti,Mo) (C,N) with a binder component of Co, Ni and/or Fe.

The invention relates further to a method of making such a sintered body.

A sintered body or part of the aforedescribed type can be used especially as a cutting insert in material removal machining operations.

It is the object of the present invention to provide a sintered body with improved cutting characteristics. In addition, a method for making such a sintered body which enables a shorter processing time and has an effect upon the lattice structure to the deepest possible penetration depth is an object.

These objects are attained with the sintered body according to claim 1 or the method according to claim 6.

Further aspects of the invention are described in the dependent claims.

Basically, a plasma diffusion treatment, especially in the form of nitriding, nitrogen incorporation or nitrocarburization has been a long practiced method for improving steel surfaces to increase the wear resistance and corrosion resistance of the steel. More recently, titanium alloys and Stellites have been subjected to plasma diffusion treatments in order to alter the lattice structures of the boundary zones of such bodies so that a diffusion zone or one or more composite layers can be formed therein. The species which can be diffused into the surface zone can be, for example, nitrogen or carbon which do not alter the crystalographic structure of the basic material except for possible spacing changes within the crystal lattice. Upon such a diffusion layer or zone proximal to the surface, one or more composite layers can be formed which in a further phase, for example, by the formation of compounds of an element of the basic material with the species diffused into the basic material can be provided.

From the abstract in the publication JP 053 02 140 A, a cermet has been described which has a hard phase like TiCm and a binder metal such as cobalt or nickel and is treated with a gaseous nitrogen plasma atmosphere in which the plasma is produced by high frequency or microwave discharges. In this case, a nitride zone with a 10 to 500 μm thickness is formed in which TiN particles are contained with a particle size of ≦μm in a homogeneous distribution. By contrast with this system, in the sintered body of the present invention, in the zone proximal to the surface, additional lattice components are provided by the plasma activated gas phase whose grain sizes are of the order of the magnitude of those of the lattice components which usually comprise the hard material fraction of the hard metal and cermet. The materials contained in the plasma activated gas phase and which diffuse into the surface can be nitrogen, carbon, boron and metals which have been excited to a plasma state. By contrast with purely thermally activated gas phases, the sintered body which is subjected to a plasma-activated gas phase has a significantly more deeply influenced surface zone. The plasma-activated gas phase enables, depending upon the gas composition of the plasma also a cleaning and/or reduction of the surface-proximal zone, an improvement in the surface roughness in the sense of a smoothing of the surface, as well as the development of new lattice components (phases) and their organization in the surface-proximal zone. The composition of the surface-proximal zone can, according to the invention, also improve the adhesion of subsequently applied layers of carbides, nitrides, oxides, borides or carbon or combinations of these materials. The plasma-activated treatment also enables an effect on the binder phase in that, for example, plasma-activated nitrogen can result in the formation of cobalt-nickel nitrides or iron nitrides which is not possible with thermally activated nitrogen.

Preferably the sintered body according to the invention can have a boundary zone [surface zone] in which the migration and/or diffusion, materials from the plasma-activated gas phase or compounds formed therefrom can be contained. The depth of the affected boundary zone or surface zone can be controllable by the choice of the process parameters temperature, pressure and treatment time in addition to the lattice homogeneity of the boundary zone to depths of up to 1200 μm.

Especially additional nitride particles with a particle size of ≧0.2 μm can develop in the boundary zone by the use of the plasma-activated gas phase.

To produce such a sintered body, a method is used in which the hard metal or the cermet are pretreated powder metallurgically and pressed to a green body which during the heating up to the sintering temperature, during the sintering or following the finished sintering is at least timewise advantageously over a time span of at least 10 min to 100 min, completely or only partially reacted in a plasma-activated gas phase under a pressure of a maximum of 3×10⁴Pa. The plasma activation can be produced by microwave discharge or by a glow discharge, whereby the glow discharge is preferably produced by means of a pulsed process in which the sintered body or sinterable body is connected as the cathode to which a pulsed direct current is applied.

Preferred direct current voltages lie between 200 to 900 volts. The direct voltage can drop, during the intervals between pulses, either to zero volts or to a residual direct current voltage which is equal to or greater than the lowest ionization potential of the gases participating in the plasma and which is a maximum however, of 50% of the maximum value of the pulsed direct current voltage. To the extent that a residual direct current voltage is maintained during the pulsed intervals, its ratio to the maximum direct current voltage which is applied should be 0.02 to 0.5. The duration of the period of the pulsed direct current is voltage lies between 20 μs and 20 ms. The ratio of the pulse duration to the pulse interval is between 0.1 to 0.6.

As already indicated, the plasma-activated gas phase should contain nitrogen, carbon, boron or also plasma activatable metals, compounds or mixtures of the aforementioned materials or also precursors thereof.

According to a further feature of the method of the invention, before admitting a reactive gas of a reactive gas mixture to the treatment chamber, the treatment chamber is supplied with an inert gas atmosphere, especially of a noble gas and/or a chemical reducing agent, preferably hydrogen. Chemically nonreactive substances like, for example, argon serve to clean the surface, after which in a further method step, the plasma-activated gas phase is admitted to the chamber to release the substances into the surface-proximal layer with the targeted migration process and diffusion process. The hydrogen contained in the gas phase serves to excite a reduction process on the surface to break down oxide coatings and scale.

For the treatment of the sintered body or the sinterable body in the plasma-activated gas phase temperatures above 900° C. to 1350° C. are selected.

Further advantages of the invention will be described in greater detail based upon the examples given hereinafter as well as with reference to the drawing. In the drawing:

FIGS. 1 to 4 show respective photo micrographs of sintered bodies which have been subjected to a plasma-activated gas phase (shown at a) by contrast with comparative samples respectively shown at b).

EXAMPLE 1

Initially two sintered hard metal bodies of the type WC—Ti(CN)—Co with identical chemical compositions (respectively 40% by weight W, 25.5% by weight Ti, 9% by weight Ta, 0.5% by said Nb, 7% by weight C, 3% by weight N and 15% by weight Co) is treated at a subeutectic temperature of 1350° C. at 300 mbar for 20 min in a nitrogen atmosphere, whereby the first hard metal body is treated with a plasma while the second is not. In both samples, a so-called graded layer is formed which is enriched in titanium carbonitride with simultaneous displacement of a WC component toward the interior of the sample. As can be seen from FIG. 1 a by contrast to FIG. 1 b, however, the nitrogen affected zone with the sample treated with the nitrogen plasma (FIG. 1 a) is significantly larger than the zone of the sample which has been only thermally treated but has not been subjected to a plasma-activated gas phase.

EXAMPLE 2

A sintered WC—Ti(C,N)(Co) hard metal body of the composition 60.5% by weight W, 16% by weight Ta, 0.3% by weight Nb, 7% by weight C and 1.2% by weight N as well as 10% by weight Co, is treated at a temperature of 1350° C. at 300 mbar along its side surfaces and upper side with a nitrogen plasma while the underside is not treated with this plasma. FIG. 2 a shows a photo-micrograph of the hard metal body upperside from which it can be seen that a 25 μm thick Ti(C,N) rich layer has developed without WC particles, as appears clear in the image, while the photomicrograph of the underside according to FIG. 2 b practically shows no effect from the nitrogen which in this region was only thermally activated and to which the underside was exposed.

EXAMPLE 3

Two sintered hard metal bodies of the composition 40% by weight W, 25.5% by weight Ti, 9% by weight Ta, 0.5% by weight Nb, 7% by weight C, 3% by weight N and 15% by weight Co are annealed at a subeutectic temperature of 1250° C. at 150 mbar N₂ for 60 min, whereby the first hard metal body is treated with plasma-activated nitrogen and the second with nitrogen which is only thermally activated in the gas atmosphere. The plasma treated sample shows in FIG. 3 a a clear displacement of the WC up to a depth of 1200 μm. Up to a depth of about 20 μm, a Ti(C,N) enrichment can be recognized in FIG. 3 a. The hard metal body treated only with a thermal nitrogen atmosphere shown by comparison in FIG. 3 b only has an effect to a depth of 5 μm in the boundary zone.

EXAMPLE 4

Two sintered hard metal bodies of the composition 60.5% by weight W, 16% by weight Ti, 5% by weight Ta, 0.3% by weight Nb, 7% by weight C and 1.2% by weight N and 10% by weight Co are annealed at a subeutectic temperature of 1250° C. at 150 mbar N₂ for 60 min whereby the first body is annealed in a plasma-activated gas phase while the second is annealed in a gas phase which is only purely thermally activated. The plasma treated sample shows a nitride layer with a thickness of 50 μm and a 40 μm thick zone thereunder with a reduced WC proportion (see FIG. 4 a). The body which has been subjected only to a thermally activated nitrogen gas phase, by contrast, shows only a 5 μm thick nitride layer and an underlying zone which is less than 5 μm thick.

The aforedescribed examples show that in the treatment of a sintered body in a plasma-activated gas phase, a targeted lattice inhomogeneity is created and/or a composite layer is produced which improves the characteristics of the body in use like its cutting stability, life and reduced tendency to reactions by comparison with other bodies in machining processes.

Preferably, the plasma activation is produced by a glow discharge, especially by means of a pulsed process in which the development of continuous arcs can be avoided. The plasma activation need not be maintained over the entire treatment duration. The gas pressure is held in a range up to 300 mbar in which the plasma state is attainable, i.e. the plasma can be ignited and maintained. By the choice of the treatment temperature or its limits, it can be insured that in the regions within the interior of the body, there is no significant thermal effect so that the lattice structure in the interior of the body retains its original form and only the surface-proximal zones are effected. By partial covering [masking] of the surfaces of the sintered body or the device or its coatings, it can be insured that no plasma will without affect particular regibns and that so-called plasma seems will not be formed. Slight lattice modifications in these regions will be influenced only by the gas atmosphere and the process parameters which are (selected) but not by the plasma which will only affect lattice modifications in the surface proximal boundary regions which are exposed.

If necessary or desirable, a pretreatment with a glow discharge can precede the plasma-activated gas phase to clean the surface of the body. Alternatively, also prior to the plasma-activated gas phase treatment, a treatment of the body can be carried out in which the body is treated with a gas phase comprising a chemical reducing agent.

By the effect of the plasma, in the surface-proximal boundary zones, a new phase is produced which cannot be formed without the plasma activation. With the method of the invention, therefore, an altered phase composition can be produced in the surface-proximal boundary regions as well as in a deeper penetration zone effecting the lattice structure and by the choice of the process parameters, a desired lattice structure inhomogeneity can be produced. This, in addition to a uniformly generated smoothing or roughening of the surface, the latter in connection with desired coating techniques, provides by comparison with sintered bodies known from the state of the art, significant advantages. 

1. A sintered body comprised of a hard metal, especially a hard metal based on WC with a Co, Ni and/or Fe binder proportions based on hard metal, or a cermet, especially based on a (Ti,W)(C,N) or (Ti,Mo)(C,N) with binder components of Co, Ni and/or Fe, characterized in that the sintered body during a heating up, during the sintering or after completion of finish sintering is treated at least timewise, preferably over a time span of at least 10 min to 100 min completely or only partially with a plasma-activated gas phase at a pressure of a maximum of 3×10⁻⁴ Pa.
 2. The sintered body according to claim 1, characterized in a surface proximal boundary zone which contains by migration and/or diffusion material from the plasma-activated gas phase or compounds produced therefrom.
 3. The sintered body according to claim 1, characterized in that the sintered body is treated in a plasma-activated gas phase which contains nitrogen, carbon, boron, metals, compounds or mixtures thereof or corresponding precursors.
 4. The sintered body according to claim 1 characterized in that the surface-proximal zone of the sintered body, contains nitride particles of a particle size ≧0.2 μm.
 5. The sintered body according to claim 1 characterized by a lattice gradient of the sintered body from the surface toward the interior of the body.
 6. A method for producing a sintered body from a hard metal or a cermet according to claim 1 characterized in that the body pressed to a green body and subjected to treatment in a powder metallurgical manner during the heating up to the sintering temperature, during the sintering or after subsequent finish sintering is at least timewise, preferably over a time span of at least 10 min to 100 min, completely or only partially treated with a plasma-activated gas phase at a pressure of a maximum of 3×10⁻⁴ Pa preferably at a temperature between 900° C. to 1350° C.
 7. The method according to claim 6, characterized in that the plasma activation is carried out by microwaves and/or by a glow discharge, preferably by means of a pulsed process in which the sintered body is connected as a cathode and to which a pulsed direct current voltage is applied.
 8. The method according to claims 6, characterized in that the plasma-activated gas phase contains nitrogen, carbon, boron, metals, compounds or mixtures thereof.
 9. The method according to claim 6, characterized in that prior to admission of a reactive gas or a reactive gas mixture, the gas phase contains an inert gas, especially a noble gas and/or a chemical reducing agent, especially hydrogen.
 10. The method according to claim 6, characterized in that during the treatment in the plasma-activated gas phase, parts of the surface of the substrate body are masked. 